Why is the concentration of [OH-] not equal to the square root of Kw?

[disneychannel]

So I thought if I have the value of Kw and I need to find the concentration of [OH^-] then all I would have to do is square root the Kw. But in this example, it is not the case why?

K_w at 50^oC is 5.48x10^-14. Calculate OH^- concentration

so i just square rooted 5.48x10^-14 and got 2.34 x10^-7. Shouldn't this be right?

 

[Borek]

For a pure water at 50°C that would be a correct approach. Is there some additional information in the question?

 

[disneychannel]

No. It just states "calculate the [OH] concentration from the above solution at 50 degrees" But, I am pretty sure that would not make a difference. Correct?

 

[sjb-2812]

No. It just states "calculate the [OH] concentration from the above solution at 50 degrees" But, I am pretty sure that would not make a difference. Correct?

What is "the above solution"?

 

[ycheff]

K_w= [H⁺][OH^-] by definition.
If [H⁺]=[OH^-] then [OH^-]=Sqrt (K_w)